A lithium secondary battery and a lithium-ion secondary battery (hereinafter referred to as a secondary battery) have features of being small and having large capacity, and are widely used as a power source of a cellular phone, a laptop computer or the like.
Currently, as an active material for a secondary battery, lithium cobalt oxide (LiCoO2) is mainly used for a positive electrode. However, since the safety in a charged state of LiCoO2 is not always sufficient and the price of a Co raw material is high, the search for a new alternative active material for a secondary battery has been promoted.
Meanwhile, as a method for increasing an energy density of a secondary battery, a method for increasing an operating potential of a secondary battery is effective. In a secondary battery using lithium cobalt oxide or lithium manganese oxide as an active material for a positive electrode, an operating potential is a 4 V class (average operating potential=3.6 to 3.8 V: vs. lithium potential). This is because a generated potential is defined by a redox reaction of a Co ion or a Mn ion (Co3+←→Co4+ or Mn3+←→Mn4+).
In contrast, it is known that a 5 V-class operating potential can be realized by using, for example, a compound having a spinel structure (spinel compound), in which Mn of lithium manganese oxide is replaced by Ni or the like, as an active material. Specifically, it is known that a potential plateau is shown in a region of 4.5 V or more by using a spinel compound of LiNi0.5Mn1.5O4 or the like (Patent Literature 1). Mn exists in a tetravalent state in such a spinel compound, and an operating potential is defined by redox of Ni2+←→Ni4+ instead of redox of Mn3+←→Mn4+. Moreover, it is known that, similarly, Li[CoMn]O4, Li[Fe0.5Mn1.5]O4, Li[CrMn]O4, Li[CuxMn2-x]O4 or the like charges and discharges at a potential of 4.5 V or more with respect to Li metal. Li[Ni0.5Mn1.5]O4 has capacity of 130 mAh/g or more and an average operating voltage of 4.6 V or more with respect to Li metal. In the case of using Li[Ni0.5Mn1.5]O4, an energy density that can be stored in a positive electrode is higher than that of LiCoO2. Furthermore, a spinel compound of Li[Ni0.5Mn1.5]O4 or the like can obtain an energy density of 90% or more even at a low temperature, for example, at −20° C. A spinel compound can be used at a wide range of temperatures and charge-discharge rates because of its high ion conductivity.
Examples of other active materials for a secondary battery with a high energy density include composite oxides having a layered structure mainly containing Ni, such as LiNiO2 and Li[Ni0.8Co0.2]O2. Although these materials have a feature of having high discharge capacity of about 200 mAh/g, its crystal stability when being charged is low and reliability of a battery in a charged state remains a problem. In addition, since these materials have a charge-discharge range of about 3 V to 4.3 V and a region of 3.8 V or less is especially large, these materials have a low potential compared to a secondary battery using LiCoO2.
Furthermore, as other active materials for a secondary battery with a high energy density, active materials having a layered structure, such as Li[CrxLi(1/3-x/3)Mn(2/3-2x/3)]O2 and Li[NixLi(1/3-2x/3)Mn(2/3-x/3)]O2, are reported (Non Patent Literature 1, Non Patent Literature 2). These materials are represented by a form of LiMO2, and are materials having a layered structure of a Li layer, an M layer, and an O layer. Since these materials have a charge termination voltage of about 4.8 V, which is higher than that of other active materials having a layered structure, these materials have a feature of having a high energy density. However, although these materials can obtain high capacity of 200 mAh/g or more at a high temperature of 45° C. and a low charge-discharge rate of 0.025 C, capacity is decreased by about 10 to 30% at a low temperature of 20° C. and a high charge-discharge rate of 0.5 C or more. It is supposed that this is due to low ion conductivity and electron conductivity of a layered structure containing Mn.
Furthermore, as another technique, a technique in which two kinds of active materials for a secondary battery are mixed to be used is reported. For example, Patent Literature 2 discloses a mixture of LiMn2O4 and Li(NixCo1-x)O2 (0≦x≦1). Moreover, Patent Literature 3 discloses a mixture of a spinel material of LiNi0.5Mn1.5O4 or the like and a material of Li[Ni0.33Li0.1Mn0.57]O2 or the like.